Stephan Haas is engaged in theoretical research on the physics of strongly correlated electrons in solids. His recent interests include microscopic modeling and phenomenology of the high-temperature superconductors, in particular the analysis of dynamical spectra and phase diagrams. He is also using a range of numerical approaches to study low-dimensional antiferromagnets, spin ladders, spin-Peierls systems, and other quantum magnets. Furthermore, his research extends to disordered quantum systems, including random-exchange antiferromagnets, dirty superconductors, and systems with impurity-induced spin and charge textures.